Along with progress in the digitization of protective relaying devices, protective relaying devices configured to carry out protection calculation by exchanging instantaneous values of electrical quantities sampled by a plurality of protective relaying devices between the devices by using counter-transmission are in wide use. A representative example of the above is a current differential protective relaying device for protection of an electric power transmission line.
In a general current differential protective relaying device, i.e., in each of protective relaying devices installed at both ends of an electric power transmission line, an instantaneous value of electrical quantity data sampled by each of the protective relaying devices arranged at both the terminals is cyclically transmitted to the protective relaying device at the opposite terminal at regular intervals of 30 electrical degrees, and two electrical quantity data items sampled at the same time are compared with each other, whereby calculation of obtaining a differential current is carried out.
A sampling synchronization control method of sampling electrical quantities at the same time is described in Jpn. Pat. Appln. KOKAI Publication No. 50-49645 (hereinafter referred to as “Pat. Document 1”. FIG. 10 is a view showing the principle of the sampling synchronization control. Assuming that a time necessary for a first protective relaying device to receive data of a specific position on the transmission format of a second protective relaying device from a specific position (sampling timing 1) on the transmission format of the first protective relaying device is TM, a time necessary for the second protective relaying device to receive data of a specific position on the transmission format of the first protective relaying device from a specific position (sampling timing 2) on the transmission format of the second protective relaying device is TF, a transmission delay time of transmission (hereinafter referred to as “up-transmission”) from the second protective relaying device to the first protective relaying device is Td1, a transmission delay time of transmission (hereinafter referred to as “down-transmission”) from the first protective relaying device to the second protective relaying device is Td2, and a sampling timing synchronization error between the two protective relaying devices is ΔT, the relationships between TF, TM, and ΔT can be expressed by formula (1).TM=Td1−ΔT,TF=ΔT+Td2 and thusΔT=(TF−TM+Td1−Td2)/2  (1)
Further, TF and TM are measurable in the protective relaying device, and hence, assuming that the transmission delay times Td1 and Td2 are equal to each other because the same transmission route, and transmission device are used, the sampling timing is controlled such that a condition expressed by following formula (2) is obtained, whereby sampling operations at the same time are enabled (see for example, Pat. Document 1).TF−TM=0  (2)
Further, in a method of Jpn. Pat. Appln. KOKAI Publication No. 2007-68325 (hereinafter referred to as “Pat. Document 2”, a concept that sampling synchronization is to be controlled by paying attention to the fact that a normal load current is a passing current, causes no phase difference between both ends, and a differential current becomes 0 is described.
In the sampling synchronizing system shown in Pat. Document 1, there is a precondition that the transmission delay time of the up-transmission and transmission delay time of the down-transmission must be equal to each other. Accordingly, in the prior art, the target value of “TF−TM” is generally set to “0” as shown by formula (2). However, in practice, even when the same transmission route and transmission device are applied to the up-transmission and down-transmission, variation in the transmission delay time between the up-transmission and down-transmission is caused, i.e., a difference is caused between the transmission delay time of the up-transmission, and transmission delay time of the down-transmission because of data buffering of the transmission device, transmission timing, and the like.
Although the variation in transmission delay time between the up-transmission and down-transmission generally leads to a certain transmission delay time after power-startup of the transmission device, there is the possibility of a transmission delay time difference occurring between the up-transmission and down-transmission and, even a case where the transmission delay time difference becomes about 200 μS is observed.
A sampling synchronization error ΔT is caused by the transmission delay time difference. In the case of a current differential protective relaying device, the sampling synchronization error ΔT appears as a differential current error with respect to the passing current I. The differential current error in the above-mentioned case where the transmission delay time difference is 200 μS becomes about 6% with respect to the passing current I, thereby hindering realization of a current differential relaying device with high sensitivity.
Further, although the method shown in Pat. Document 2 is theoretically possible, an actual technique to put it into practice is not mentioned, and there is thus a problem in synchronizing the sampling timings with high accuracy.
Under these circumstances, it is desired to provide a protective relaying device having a function of synchronizing sampling timings with each other with high accuracy even when there is a difference between the up-transmission delay time and down-transmission delay time in the method of controlling sampling synchronization by counter-transmission.